1. Field of the Invention
The present invention relates to a hydraulic shock absorber.
2. Description of the Related Art
Generally, in a hydraulic shock absorber, there has been provided a structure in which into an oil chamber of a cylinder attached to one of a vehicle body side and an axle side, a piston rod attached to the other of the vehicle body side and the axle side is inserted. The oil chamber of the cylinder is divided into a piston side oil chamber and a rod side oil chamber by a piston provided in a leading end portion of the piston rod, and an oil reservoir chamber compensating a volumetric capacity of the piston rod moving forward and backward to the oil chamber of the cylinder is communicated with the oil chamber of the cylinder.
Further, a hydraulic shock absorber described in Japanese Patent Application Laid-Open (JP-A) No. 2001-263409 (Patent Document 1) is structured such that a piston side oil chamber and an oil reservoir chamber are divided by a partition wall member. A compression side damping force generating device is provided in an oil passage communicating the piston side oil chamber and the oil reservoir chamber which are provided in the partition wall member. The partition wall member is provided with a communication passage communicating the piston side oil chamber and the oil reservoir chamber, and the piston rod is provided with a needle which is fitted to the communication passage after the piston rod is compressed at a fixed stroke.
Accordingly, in the hydraulic shock absorber described in Patent Document 1, if the piston rod moves forward to a far side of the oil chamber of the cylinder in a compression side stroke, and the needle provided in the piston rod is fitted to the communication passage of the partition wall member, the communication between the piston rod and the oil reservoir chamber by the communication passage is shut off gradually. Therefore, the oil pressurized by the piston passes through the compression side damping force generating device provided in the partition wall member in large quantities gradually, and the compression side damping force generating device generates a great damping force gradually. In other words, the damping force is changed according to a stroke of the compression side stroke, and a position dependency of the compression side damping force is indicated.
Further, a hydraulic shock absorber described in Japanese Patent Application Publication (JP-B) No. 39-22646 (Patent Document 2) is structured such that a piston side oil chamber and an oil reservoir chamber are divided by a partition wall member, and compression side and extension side damping force generating devices are provided in an oil passage communicating the piston side oil chamber and the oil reservoir chamber which are provided in the partition wall member. Further, through-holes communicating the piston side oil chamber of the cylinder with a rod side oil chamber of the cylinder via a bypass oil passage provided around the cylinder are provided at a plurality of positions in an axial direction of the side wall of the cylinder. Each time when the piston rod extends and retracts at a fixed stroke, the piston opens and closes each of the through holes.
Accordingly, in the hydraulic shock absorber described in Patent Document 2, as the piston rod moves forward to a far side of the oil chamber of the cylinder in a compression side stroke, and the piston provided in the piston rod passes through each of the through-holes provided in the axial direction of the side wall of the cylinder, the through-holes directly communicating the piston side oil chamber with the bypass oil passage are reduced. An amount of oil flowing out to the bypass oil passage and the rod side oil chamber via the through-holes communicating with the oil chamber from the piston side oil chamber is reduced gradually due to a pressure application which the piston applies to the piston side oil chamber. In other words, the amount of oil passing through the compression side damping force generating device of the partition wall member is increased gradually, and the compression side damping force generating device generates a great damping force.
On the other hand, in an extension side stroke in the hydraulic shock absorber described in Patent Document 2, as the piston rod comes out of the far side of the oil chamber of the cylinder, and the piston provided in the piston rod passes through each of the through-holes provided in the axial direction of the side wall of the cylinder, the oil in the rod side oil chamber flows into the piston side oil chamber in large quantities gradually through the passage holes which come to communicate with the piston side oil chamber from the bypass oil passage, due to the pressure application which the piston applies to the rod side oil chamber. In other words, the amount of oil passing through the extension side damping force generating device of the partition wall member is reduced gradually, and the damping force generated by the extension side damping force generating device becomes smaller.
Therefore, in the hydraulic shock absorber described in Patent Document 2, in both the compression side stroke and the extension side stroke, the damping force is changed according to the strokes, and the position dependency of the damping force is indicated.
In the hydraulic shock absorber described in Patent Document 1, the damping force generated by the compression side damping force generating device provided in the partition wall member according to the stroke of the compression side stroke is based on the amount of oil passing through the compression side damping force generating device which is provided in the partition wall member. Further, the amount of oil passing through the compression side damping force generating device which is provided in the partition wall member depends on an annular area obtained by subtracting a cross-sectional area of the needle which is fitted to the communication passage provided in the partition wall member, from a cross-sectional area of the piston rod which goes into the cylinder, and is extremely small. Accordingly, the damping force generated by the compression side damping force generating device which is provided in the partition wall member is small, and it is impossible to obtain a great change of the damping force according to the stroke, by this compression side damping force generating device.
In the hydraulic shock absorber described in Patent Document 2, the damping force generated by the compression side damping force generating device and the extension side damping force generating device which are provided in the partition wall member according to the strokes of the compression side stroke and the extension side stroke, is based on the amount of oil passing through the compression side damping force generating device and the extension side damping force generating device which are provided in the partition wall member. The amount of oil passing through the compression side damping force generating device and the extension side damping force generating device which are provided in the partition wall member depends on a cross-sectional area of the piston rod which goes into and goes out of the cylinder, and is small. Accordingly, the damping force generated by the compression side damping force generating device and the extension side damping force generating device which are provided in the partition wall member is small, and it is impossible to obtain a great change of the damping force according to the stroke by the compression side damping force generating device and the extension side damping force generating device.
In this case, in the hydraulic shock absorbers described in Patent Documents 1 and 2, in order to increase the damping force generated by the compression side damping force generating device or the extension side damping force generating device provided in the partition wall member, and increase the change of the damping force according to the compression side or the extension side stroke of the damping force, it is necessary to increase a resistance of the compression side damping valve constituting the compression side damping force generating device, or the extension side damping valve constituting the extension side damping force generating device. However, in this case, it is necessary to apply a high sealing performance to the cylinder, and there is an inconvenience that a dispersion of the damping force with respect to a piston speed becomes large.